The present invention relates to a magnetic recording medium and a manufacturing method thereof, and more specifically to a magnetic recording medium and a manufacturing method thereof, according to which an increase in coercivity and a reduction in noise are simultaneously realized, and moreover the manufacturing cost is reduced.
Various magnetic layer compositions and structures, nonmagnetic foundation layer materials and so on have been studied and proposed for magnetic recording media, for which high recording density and low noise are demanded. In particular, in recent years, there have many proposals for ‘granular magnetic layers’, which are magnetic layers having a structure in which magnetic crystal grains are surrounded by a nonmagnetic nonmetallic substance such as an oxide or a nitride.
For example, Japanese Patent Application Laid-open No. 8-255342, it is proposed to form a nonmagnetic film, a ferromagnetic film and a nonmagnetic film in this order and then to carry out heating treatment, thus forming a granular magnetic layer (recording layer) in which ferromagnetic crystal grains are dispersed in a nonmagnetic film, whereby noise can be reduced. An oxide or nitride of silicon or the like is used for the nonmagnetic films in this case.
Moreover, U.S. Pat. No. 5,679,473 describes depositing a recording layer by RF sputtering using a CoNiPt target to which an oxide such as SiO2 has been added in advance, whereby a granular magnetic layer having a structure in which magnetic crystal grains are surrounded by a nonmagnetic oxide and are separated from one another can be formed, and hence high coercivity (Hc) and low noise can be realized.
Moreover, Japanese Patent Application Laid-open No. 2001-101651, there is disclosed a granular magnetic layer that is constituted from an alloy having Co as a principal component thereof and having an hcp structure and 3 to 50 mol % of an oxide or nitride, and for which an axis of easy magnetization is oriented in a direction approximately parallel to the substrate surface, and it is stated that an increase in coercivity and a reduction in noise are possible with this granular magnetic layer.
Furthermore, Japanese Patent Application Laid-open No. 2000-276729, an example is reported in which a recording layer is constituted from a granular magnetic layer in which a plurality of ferromagnetic crystal grains comprising an alloy having a composition such as Co80Pt15Cr5 are arranged in an SiO2 matrix, whereby a magnetic recording medium having good overwrite properties can be realized.
With such granular magnetic layers, it is thought that a nonmagnetic nonmetallic grain boundary phase physically (spatially) separates the magnetic grains from one another, and hence the magnetic interaction between the magnetic grains is reduced, and thus the formation of zigzag magnetic domain walls occurring at recording bit transitional regions is suppressed, whereby low noise can be obtained.
With CoCr-type metallic magnetic films that have been used in the past, the deposition is carried out at high temperature, and hence the Cr precipitates out from the Co-based magnetic grains and becomes segregated at the grain boundaries, which reduces the magnetic interaction between the magnetic grains; however, in the case of a granular magnetic layer, a nonmagnetic nonmetallic substance is used as a grain boundary phase, and hence segregation occurs more easily than with the Cr in a conventional CoCr-type metallic magnetic film, and thus isolation of the magnetic grains from one another can be promoted relatively easily. Moreover, in a conventional CoCr-type metallic magnetic film, to secure sufficient Cr segregation, it is essential to set the substrate temperature to at least 200° C. during the film deposition, whereas in the case of a granular magnetic layer, there is an advantage that the nonmagnetic nonmetallic substance can be made to segregate at the grain boundaries even in the case that the magnetic layer is deposited without heating the substrate.
However, with magnetic recording media having a granular magnetic layer, there is a problem in that to realize the desired magnetic properties (in particular high coercivity Hc), it becomes necessary to add a relatively large amount of Pt, which is expensive, to the Co alloy. For example, with the magnetic recording medium described in above-mentioned U.S. Pat. No. 5,679,473, to realize an Hc of approximately 2400 Oe, a large amount of Pt of 11 at % is necessary, whereas to realize approximately the same Hc with a conventional CoCr-type metallic magnetic film, adding at most 5 at % of Pt is sufficient. As the density of magnetic recording has increased in recent years, a very high Hc of 3000 Oe or more has come to be demanded of magnetic recording media, and in this situation there is a problem that with granular magnetic layers, for which a large amount of expensive Pt is required to increase the Hc, the manufacturing cost rises.
Moreover, as the recording density is increased, further reductions in medium noise are demanded, and hence there is an increasing need to finely control the microstructure of granular magnetic layers, for example the magnetic crystal grain size and the segregation structure. For example, Japanese Patent Application Laid-Open No. 2002-015417, it is disclosed that by forming a nonmagnetic intermediate layer of a nonmagnetic metal or alloy having an hcp crystal structure between a granular magnetic layer and a nonmagnetic foundation layer, it is possible to realize high Hc and low medium noise; however, to further reduce noise, crystal design on an atomic level, i.e. control of the state of orientation and the misfit amount of the crystal lattice between the respective layers, is considered to be necessary.
In view of the problems described above, it would be desirable to provide a magnetic recording medium and a manufacturing method thereof, according to which an increase in coercivity and a reduction in noise are simultaneously realized, and moreover the manufacturing cost is reduced.